An adder circuit inhibiting overflow is provided. A first memory, a second memory, a third memory, and a fourth memory are included. A step of supplying first data with a sign to the first memory and supplying the first data with a positive sign stored in the first memory, to the second memory; a step of supplying the first data with a negative sign stored in the second memory, to the third memory; a step of generating second data by adding the first data with a positive sign stored in the second memory and the first data with a negative sign stored in the third memory; and a step of storing the second data in the fourth memory are included. When the second data stored in the fourth memory are all second data with a positive sign or all second data with a negative sign, all the second data stored in the fourth memory are added.